What you described above - creating better opportunities to take care of them before they take care of you, using the tools the game gives you - is not cheating. It's smart gameplay. Same for fashioning an exit before entering a potentially dangerous area. Same for putting down climb-able escape blocks before any (obviously a trap) 2+ block drops into rooms you can't see. These are S-M-R-T smart tactics.
Sleeper volumes are actually volumes, not trigger points (though I suppose it's possible to set up a 1x1x1 volume to act as a trigger). For the loot room at Red Mesa, there is a volume that covers the room itself (but 
not tall enough to cover the hatch at the top, so you can open the hatch without triggering the volume) as well as the area outside the vault door.
There is another sleeper volume which wraps around behind the loot room. Surpisingly, only the 2nd volume appears to have actual sleepers in it. They both have the same Group ID, though, which I assume means that both volumes apply to the same sleepers.
Only the vault-room volume (the one bordered in green) is set to "Attack" on activation. The other is "Active" which I think activates their hearing, but does not automatically wake them up. So if you somehow entered this area from behind the room (in the blue-bordered volume), you could conceivably stealth-kill the sleepers before you enter the main (green-bordered) vault volume. Of course the One True Path leads you into the vault-room volume which sets all the zombies to attack immediately.
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